Rotary flexure bearing

ABSTRACT

This rotary flexure bearing is a monolithic structure with inner and outer hubs connected by unique rotary compound flexure stages. These rotary compound flexure stages provide the angular compliance required for bearing rotation as well as compliance for flexure foreshortening while holding a constant axis of rotation with great precision over the entire range of motion. This design offers large angular displacement, low operating stress, low operating torque, and high stiffness in the five noncompliant degrees of freedom. The rotary flexure bearing described herein has applications in precision mechanics, particularly opto-mechanics. Specific applications include but are not limited to wafer and reticle alignment stages used in microlithography systems, mirror pointing and scanning mechanisms used in tactical and spaceborne systems, as well as flip-in mechanisms used in multiple field of view optical systems.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not Applicable

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

REFERENCE TO SEQUENCE LISTING

Not Applicable

BACKGROUND OF THE INVENTION

This invention generally relates to a flexure based rotary guide bearing. Flexures have been used successfully in simple and inexpensive as well as complicated and expensive motion systems for centuries. Since they operate by bending, not rolling or sliding, flexures have the inherent advantage of friction-free motion. This key feature allows engineers to build positioning systems with nearly unlimited precision and accuracy. They are also easy to design and fabricate. However, even with these desirable attributes there are very few flexure based bearings available commercially. Therefore, it is common practice for an engineer to design custom flexure systems while developing a new mechanism. The rotary flexure bearing described herein is particularly well suited for integration into precision motion systems and opto-mechanical mechanisms where friction-free rotation over a limited angular range is required. It has integrated mounting and registration features that are concentric, and perpendicular, to the axis of rotation. It also has a configuration that scales easily without compromising the operating principle, making this concept a convenient basis for a family of rotary flexure bearings. In addition to applications in precision mechanics, this bearing can be used in hostile operating conditions (extreme temperatures, extreme changes in temperature, vacuum, corrosive environment, contaminated environment . . . ) that normally prohibit use of conventional bushings, rolling element bearings, or gas lubricated bearings.

SUMMARY OF THE INVENTION

Motivation for the proposed rotary flexure bearing is partially based on the limited availability of high quality rotary flexure bearings. The most popular offering has remained unchanged since 1955 when it was first developed by the Bendix Corporation. However, there are some shortcomings associated with these Bendix flex-pivots that engineers have had to overcome or accept.

Four of the shortcomings are:

1) Since they do not have integrated mounting features, the engineer must design and build custom hardware to attach the flex-pivot to the mechanism. The manufacturing and assembly tolerances associated with this additional hardware will increase the coaxial error between the desired and actual axis of rotation.

2) The flex-pivots are made of multiple pieces of 400 series stainless steel that are brazed together. This choice of material and fabrication technique favors mass production but limits application to environments benign enough for 400 series stainless steel, the brazing material, and the galvanic potential of the stainless steel-to-brazing material interface. Failure of any brazed joint will result in catastrophic failure.

3) The operating principle of these flex-pivots is based on beams bridging the gap from a fixed base to a free section. The beams cross each other at 90° and when the free section is rotated the beams bend creating an axis of rotation where they cross. However, the shape of these beams is a function of angular deflection, so the location of the axis of rotation is also a function of angular deflection.

4) In addition to a constantly changing axis of rotation and multiple single point failure sites, the radial stiffness of this nonsymmetrical beam arrangement varies with radial vector angle and direction.

Readily available high quality materials and manufacturing techniques that were not available in 1955 can be used to create high performance alternatives to the Bendix flex-pivot. The proposed rotary flexure bearing addresses the shortcomings listed above as follows:

1) The proposed rotary flexure bearing has integrated mounting flanges. In addition to a screw-hole pattern, centering and clocking features are machined into the mounting flanges during flexure fabrication. This makes the flanges concentric with the axis of rotation and eliminates the need for user designed mounting hardware.

2) The proposed rotary flexure bearing is a seamless monolithic structure. Conventional machine tools and the wire EDM process are used to fabricate this flexure bearing, which permits the use of any application appropriate metal.

3) The operating principle of the proposed rotary flexure bearing is based on multiple compound flexure stages that have been arranged into concentric circular segments. The resulting system is a rotary flexure bearing that has a fixed axis of rotation. Three compound flexure stages will be used to illustrate the principle of operation in the detailed description section.

4) The symmetrical design of the proposed rotary flexure bearing yields a consistent radial stiffness regardless of radial force vector angle or direction. SEE FIG. 4

DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1: Perspective view of the proposed rotary flexure bearing.

FIG. 2: Front view of the proposed rotary flexure bearing.

FIG. 3A: Front view of the proposed rotary flexure bearing showing patterns of dowel pin holes, mounting screw holes, and mounting pads.

FIG. 3B: Section view of the proposed rotary flexure bearing.

FIG. 4: Definition of radial force vector.

FIG. 5: FEA model of the radial flexure bearing at mid-travel.

FIG. 6: FEA model of the radial flexure bearing near end of travel.

FIG. 7: Mid-travel and rotated FEA models showed together.

DETAILED DESCRIPTION OF THE INVENTION

The arrangement of three compound flexure stages can be seen in FIG. 2. Each compound stage has four blade flexures. All twelve blade flexures are the same thickness, width, and length, therefore have the same stiffness as well. The two inner blade flexures 4 in each compound stage connect the outer hub l of the rotary flexure bearing to the regulator link 5. The two outer blade flexures 3 in each compound stage connect the three-legged inner hub 2 to the regulator link 5. Each of the three compound stages is defined by one inner blade flexure stage and one outer blade flexure stage that are nested together. The inner blade flexure stage consists of the inner blade flexures and the regulator link while the outer blade flexure stage is defined by the outer blade flexures and regulator link. The regulator link is shared by both the inner and outer blade flexure stages and allows the two stages to work together as a complete compound flexure stage. The inner blade flexure stage has two functions. The first is to serve as a pivot for the regulator link. The inner blade flexure stage guides the regulator link around the rotary flexure bearing center line/axis of rotation. The second function is to foreshortening the same amount as the outer blade flexure stage so that the system of four blade flexures in the compound flexure stage is not over constrained. The use of flexures that have the same geometry and stiffness in the inner and outer stages makes this complimentary foreshortening possible, and the complimentary foreshortening allows the compound flexure stage to operate over large angular deflections while experiencing low flexure stresses. The outer blade flexure stage rotates around the rotary flexure bearing center line/axis of rotation and also moves with the regulator link which is guided by the inner blade flexure stage. Therefore the inner and outer blade flexure stages follow the same path, experience the same angular displacement, and the same blade flexure foreshortening. SEE FIGS. 5, 6, and 7. This system of three compound flexure stages connects the outer 1 and inner 2 hubs. The maximum bearing rotation is limited by integral hard stops 6. In each compound stage, the two outer blade flexures work together as springs in parallel between the inner hub and regulator link. The two inner blade flexures work together as springs in parallel between the regulator link and the outer hub. As a system, the outer blade flexure stage and inner blade flexure stage work together as springs in series between the inner and outer hubs. When the inner hub is rotated with respect to the outer hub the regulator link also rotates around the same center line, but only half as much as the inner hub. This is because the regulator link is at the mid span of what is essentially a continuous spring bridging the gap between the inner and outer hubs. The deflection at mid-span is half the deflection at the end. This creates an inner blade-to-outer blade angular phase control that is both passive and positive. Since the blade flexures have the same stiffness and share the same loading, all twelve bend the same amount in an “S” shape. They also experience the same amount of foreshortening at the same rate of change while the inner hub rotates with respect to the outer hub. This common and simultaneous change in blade flexure length results in outward radial translation of the regulator links while the inner hub axis of rotation remains constant and collinear with the outer hub centerline. SEE FIGS. 5, 6, and 7.

Unsupported free ends of flexures are a source of instability in some multiple flexure mechanisms. The free ends are easily excited by external shock and vibration as well as the normal motion of the mechanism. The regulator link is at the free ends of the blade flexures in the compound stages used in this rotary flexure bearing design. However, the regulator link cannot freely rotate in an independent fashion like the inner or outer hubs. While the hubs are fixed, the regulator link is also fixed. When the inner hub is rotated the outer blade flexures, which move with the inner hub, rotate the regulator link which is guided by the inner blade flexures. This regulated motion is made possible by the unique blade flexure arrangement in the compound stage. If the four blade flexures were parallel they would be free to bend together at any time, and the regulator link would travel with them. Since the four blade flexures are not parallel an over constrained condition exists. The regulator link is not free to move independently, so this flexure arrangement is not subject to undesirable excitations of the flexure free ends. The controlled motion of the regulator link in this compound flexure stage helps to reject external disturbances and creates a fast settling mechanism that is very responsive to high acceleration moves.

Alignment and mounting features are shown in FIG. 3A and FIG. 3B. The counter-bored screw . holes are threaded to offer mounting options. The dowel pin and screw hole patterns are concentric to the inside and outside diameters. The front and rear faces of the hubs are relieved creating coplanar pads where the mounting screw and dowel pin holes are located. This relief provides clearance between the edges of the blade flexures and the mechanism mounting surface, which allows the blade flexures to bend and rotate without interference. The outer perimeter of the outer hub is relieved in three places, leaving three pads that define the outside diameter of the rotary flexure bearing. These three pads are finish machined by wire EDM in the same set up used to finish machine the blade flexures. This approach limits the baseline error between the centerline of the outer hub and axis of rotation of the compound flexure stages to the precision of the wire EDM machine. The outside diameter of the outer hub is controlled for a slip fit into a locating diameter that has been machined into the housing of an instrument. Alternatively, three dowel pins passing through the three outer dowel pin holes could engage a pattern of three radial slots in the mounting surface of an instrument. These same techniques could be used to align the inner hub to the mechanism. The bearing diameter can be increased to allow for a large hole through the center of the inner hub for applications requiring unobstructed passage of light or mechanics. However, if the blade flexure length is increased significantly while scaling the bearing geometry, a change in blade profile may be required to control stiffness of the blade flexures. 

1. A rotary flexure bearing made from one piece of material creating a continuous and monolithic system of inner and outer hubs connected by identical and unique radial compound flexure stages that are arranged in a radial pattern centered on the bearing axis of rotation.
 2. The inner and outer hubs of claim 1 share a common center line with the rotary flexure bearing axis of rotation.
 3. The inner and outer hubs of claim 1 contain a pattern of counter-bored and threaded mounting screw holes centered on the hub center line.
 4. The inner and outer hubs of claim 1 contain a pattern of dowel pin holes centered on the hub center line.
 5. The front and rear faces of the hubs of claim 1 have raised pads where the hubs make contact with a mechanism mounting surface.
 6. The outside diameter of the outer hub of claim 1 has raised pads that are designed to be finished in the same wire EDM machine setup used to finish the blade flexures.
 7. The inner and outer hubs of claim 1 contain features which limit the angular range of the rotary flexure bearing.
 8. The radial compound flexure stage of claim 1 has one degree of freedom.
 9. The radial compound flexure stage of claim 1 has four identical blade flexures.
 10. The radial compound flexure stage of claim 1 has an inner and outer stage.
 11. Each stage in the compound stage of claim 1 has two blade flexures.
 12. The blade flexures in each compound flexure stage of claim 1 are arranged in a radial pattern.
 13. The inner and outer stages in the radial compound flexure stage of claim 1 share a common member called a regulator link. 